1. Technical Field
The present invention generally relates to biomedical devices such as ophthalmic lenses.
2. Description of Related Art
Biomedical devices such as ophthalmic lenses made from siloxy-containing materials have been investigated for a number of years. Such materials can generally be sub-divided into two major classes, namely hydrogels and non-hydrogels. Hydrogels can absorb and retain water in an equilibrium state whereas non-hydrogels do not absorb appreciable amounts of water. Regardless of their water content, both hydrogel and non-hydrogel siloxy and/or fluorinated contact lenses tend to have relatively hydrophobic, non-wettable surfaces.
Hydrogels represent a desirable class of materials for many biomedical applications, including contact lenses and intraocular lenses. Hydrogels are hydrated, crosslinked polymeric systems that contain water in an equilibrium state. Silicone hydrogels are a known class of hydrogels and are characterized by the inclusion of a siloxy-containing material. Typically, a siloxy-containing monomer is copolymerized by free radical polymerization with a hydrophilic monomer, with either the siloxy-containing monomer or the hydrophilic monomer functioning as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed. An advantage of silicone hydrogels over non-silicone hydrogels is that the silicone hydrogels typically have higher oxygen permeability due to the inclusion of the siloxy-containing monomer. Because such hydrogels are based on free radical polymerization of monomers containing a crosslinking agent, these materials are thermosetting polymers.
In the field of biomedical devices such as contact lenses, various physical and chemical properties such as, for example, oxygen permeability, wettability, material strength and stability are but a few of the factors that must be carefully balanced in order to provide a useable contact lens. For example, since the cornea receives its oxygen supply from contact with the atmosphere, good oxygen permeability is an important characteristic for certain contact lens material. Wettability also is important in that, if the lens is not sufficiently wettable, it does not remain lubricated and therefore cannot be worn comfortably in the eye. Accordingly, the optimum contact lens would have at least both excellent oxygen permeability and excellent tear fluid wettability.
One problem associated with silicone lenses is the surfacing of silicone chains which create hydrophobic areas on the lens. This will adversely impact wettability, eye-movement and comfort to the user.
One way to alleviate this problem is by coating the surface of silicone hydrogel contact lenses with hydrophilic coatings, such as plasma coatings.
Another way to alleviate this problem is to incorporate a relatively large amount of a hydrophilic monomer such as dimethacrylamide (DMA) and/or N-vinyl-pyrrolidone in the monomer mixture. A drawback to this approach is there is potential leaching of PVP and DMA oligomers which, because of their low reactivity to relative to methacrylates, may not be covalently incorporated into the polymer network.
Accordingly, it would be desirable to provide improved biomedical devices such as contact lenses that exhibit suitable physical and chemical properties, e.g., oxygen permeability, lubriciousness and wettability, for prolonged contact with the body while also being biocompatible. It would also be desirable to provide improved biomedical devices that are easy to manufacture in a simple, cost effective manner.